The present invention relates to a bracket for orthodontics and, more particularly, to an orthodontic bracket having excellent aesthetic properties and enhanced strength as well as strong adhesion to the tooth.
In orthodontic devices for correcting irregularities of teeth, a bracket is used as a means for connecting together a wire and a band.
In actual use, the bracket is rigidly secured to the surface of a tooth which is desired to be straightened in order to apply thereto a flexural or tensile load derived from the wire. For this purpose, metallic brackets have heretofore been generally used, and recently plastic and ceramic brackets have also been put into practical use.
Metallic brackets have the advantage that they have superior strength and enable straightening force from a wire or the like to be effectively transmitted to the teeth, but they are inferior in terms of aesthetic properties. On the other hand, ceramic brackets have excellent transparency and therefore are superior from the aesthetic viewpoint, but they are generally considered to be relatively brittle and inferior in terms of adhesive properties.
Conventional plastic brackets have superior aesthetic and adhesive properties and therefore widely used for the edgewise method and the like but they are inferior in terms of strength.
Conventional plastic brackets suffer from the following clinical problems: (1) the wing portions thereof are readily fractured; (2) these brackets are apt to discolor; (3) the wire is unable to slide smoothly; (4) it is impossible to provide siamese type brackets which can be used for 21 12; and (5) it is impossible to provide torque angulation.
Plastic brackets which are now widely used are made of thermoplastic polycarbonates. Polycarbonates, which have good transparency and excellent impact resistance, are generally used as engineering plastics. However, brackets made of thermoplastic polycarbonates are insufficient for use in orthodontics because of relatively low surface hardness and somewhat low mechanical strength (i.e., tensile strength, flexural strength and compressive strength), and therefore the polycarbonate brackets do not necessarily exhibit satisfactory functions as orthodontic devices. Since the surfaces of the plastic brackets are not satisfactorily hard the bracket surfaces are likely to be scratched with toothbrushes or the like, which leads to wear and contamination (discoloration). In addition, since the mechanical strength of the conventional plastic brackets is inadequate, the wing portions thereof are readily fractured as described above. Therefore, it is difficult to use such conventional plastic materials to produce brackets having various configurations (i.e., siamese type brackets, brackets provided with torque angulation, etc.) which are needed for orthodontics.
It should be noted that typical examples of adhesives for orthodontic brackets which may be used to carry out orthodontics include 4-methacryloyloxyethyltrimellitic acid anhydride, methyl methacrylate monomers, mixed compositions of polymethyl methacrylate powders and tributylborane (TBB) [e.g., "Superbond C&B" (trade name) manufactured by Sun Medical Co., Ltd.), and mixed compositions of methyl methacrylate monomers, BisGMA, BPO and a tertiary amine (redox system type).